nixos: don't set systemConfig for stage-2

Since c4f910f55015b150bebe3d3ecdea76c1aa8bbcca, this is no longer
needed, because stage-2 is already generated with the path hard wired anyway.

nixos/modules/system/boot/loader/init-script/init-script-builder.sh

@@ -49,7 +49,6 @@ addEntry() {
       echo "#!/bin/sh"
       echo "# $name"
       echo "# created by init-script-builder.sh"
-      echo "export systemConfig=$(readlink -f $path)"
       echo "exec $stage2"
     )"